1. Field of the Invention
This invention relates to a patterning method which makes use of an atomic beam holography, and more particularly to a method of and an apparatus for forming a pattern on the surface of a semiconductor substrate and so forth.
2. Description of the Related Art
Progressively increasing attention is paid to technology for using a hologram to transfer a fine pattern on a semiconductor substrate and so forth. The pattern transferring technology by a hologram is regarded as a technique of very fine lithography in fabrication of a VLSI (very large scale integrated circuit).
Stepper technology conventionally employed for fabricating a VLSI uses optical exposure wherein a mask is contacted with a substrate to which a resist is applied to transfer a pattern. Such lithography technology requires a complicated adjusting operation because it employs many lenses for an optical system thereof. Further, the lithography technology is disadvantageous also in that one piece of dust sticking to a mask forms a fatal defect of a transferred pattern. Even with an optical lithography method wherein a reduction projection optical system is used for exposure without contact of a mask with a substrate, the problem of sticking of dust is significant and makes adjustment of the optical system difficult.
On the other hand, pattern transfer by a holographic technique is advantageous in that it does not require a complicated lens system for reproduction of a hologram and pattern transfer can be performed without contact between a mask and a substrate. Therefore, where a stepper is used, the pattern transfer by a holographic technique is not liable to be influenced by dust which causes a problem in a fabrication process. Further, since pattern information recorded on a hologram is distributed over the overall area of the hologram, even if some physical defect occurs with a portion of the hologram, this does not produce a fatal defect in a reproduced image. In other words, the pattern transfer technology which uses a hologram has advantage that it is tough against a defect.
Further, a holographic technique has a characteristic that, if the same optical system as that used upon recording is used to reproduce a hologram, otherwise possible aberration is eliminated. In this instance, the resolution of a final pattern depends on the wavelength. According to an existing stepper which employs a reduction optical system, since the pattern resolution depends on the lens aberration, lithography by a hologram has advantage that a simple optical system can be used to form a high resolution pattern at a wavelength limit when compared with lithography which employs an existing stepper in which a mask and a reduction optical system are used.
Such an optical lithography apparatus which employs holography as described above has already been placed on the market and is operating in a fabrication process on the practical use level. In fabrication of VLSI at present, formation of a pattern approximately on the sub micrometer order is required, and to this end, a light source for use with an optical lithography apparatus by a hologram exhibits gradual reduction in wavelength to a G-line (436 nm) and further to an I-line (365 nm). Recently, it is argued to form a pattern with an ultraviolet (UV) laser such as a Krk excimer laser or an ArF excimer laser.
In pattern formation by holography, the minimum resolution depends on the magnitude of a hologram dry plate and the wavelength of a light source used. Resolution dX is given by a general formula for a lens optical system: EQU dX=.lambda.L/D (1)
where .lambda. is the wavelength, L is the distance between the hologram and the lens, and D is the diameter of the hologram. Since an available hologram is limited in size, pattern resolution dX finally depends on wavelength .lambda..
Light emerging from a hologram includes zero-order light and higher-order diffraction light of the first order or higher. Normally, since zero-order light does not include phase information, not zero-order light but higher-order diffraction light is used to reproduce a pattern. This is why an off-axis optical arrangement is taken in reproduction of a hologram, and an inevitable optical arrangement when a hologram is reproduced with higher-order light by which reproduction of a pattern is performed except zero-order light by which reproduction of a pattern cannot be performed is the off-axis arrangement. The off-axis arrangement is an arrangement displaced from an optical axis.
By the way, information recorded on a hologram dry plate is phase and intensity information of wave (light) emerging from a body and is a Fourier transform of the body shape, that is, a pattern. From this, it is possible to produce a hologram of a given body shape or pattern artificially by calculation, and a binary calculation hologram by a computer was produced in 1967. Since then, a method of discrete fast Fourier transform (DFFT) has been improved, and now, a reproduced image of a good quality can be formed and also a three-dimensional (3D) pattern can be formed with a computer-synthesized hologram. In such a binary computer-synthesized hologram as described above, optical information, particularly phase information, from a virtual substance is recorded as information of "0" or "1" on a hologram with an information recording plane of the hologram divided into a finite number of cells. For example, "1" corresponds to a hole (cell) whose light transmittance is 100% while "0" corresponds to a cell whose light transmittance is 0%. A computer-synthesized hologram has such holes formed therein in accordance with phase/intensity information from a virtual substance. Japanese Patent Laid-Open No. Hei 8-286591 (JP, 08286591, A) discloses a technique wherein a computer-synthesized hologram is used and a material wave such as an ion beam, a neutral particle beam or an electron beam is passed through the hologram to project a hologram image on a resist to form a pattern corresponding to the hologram image.
Here, it is examined to improve the resolution of a pattern. In order to improve the resolution of a pattern, the wavelength of wave motion to be used should be made shorter as apparently seen from equation (1) above. In particular, for fine processing for which a wavelength shorter than the wavelength of an excimer laser such as a KrF or ArF excimer laser used at present, it is effective to use an X-ray or an electron beam having a further shorter wavelength as a light source for lithography.
Further, in holographic pattern formation of a high resolution, a wave source having a good coherence should be used as a light source. For example, it is possible to use a material wave (de Bloglie wave) of an electron beam emitted by field emission. K. Ogai, S. Matsui, Y. Kimura and R. Shimizu, in Appl. Phys. Lett. 66, 1560 (1995) disclose an example wherein an electron beam whose energy is 100 keV and whose wavelength is 0.003 nm is used to form a grating by means of a biprism which is a kind of hologram.
In order to diffract wave motion (light) of such a short wavelength as described above, it is necessary to form a hologram pattern of a size approximately equal to one wavelength. The development of the very fine pattern formation technology by electron beam exposure in recent years is so remarkable that it is possible to form a pattern of the 0.02 .mu.m order and a very fine hologram can Fax be formed corresponding to the decreased wavelength of a light source.
The limit in fine pattern formation by the existing pattern formation technology is examined for different kinds of it. Where light is used, the practically available shortest wavelength is approximately 180 nm which is that of an excimer laser, and the wavelength just mentioned is the limit in fine mask pattern formation as a sufficiently fine processing technique is available. In the X-ray lithography, a fine pattern to approximately 20 nm can be formed by utilizing an electron beam for mask pattern formation. Further, a reduction pattern formation method in which a concave X-ray mirror is used can be applied, and it seems possible to form a pattern approximately of a wavelength of an x-ray in principle. However, even if an X-ray or an electron beam is utilized as a radiation source, an existing lithography technique takes a process wherein a pattern is first transferred to an intermediate material such as an organic resist or an inorganic resist and then transferred to a substrate of Si, GaAs or the like material by such a technique as lift-off or etching. Here, resolutions intrinsic to the resist materials matter. Since the photosensitive mechanisms of the resist materials make use of a reaction that a bond of molecules is broken or molecules polymerize finally, the range of secondary electrons in a resist material determines the resolution of the resist material. The range of secondary electrons generally is approximately 5 nm, and as a result, the resolution limitation of a pattern is approximately 10 nm.
The principle of pattern formation in which a hologram is used and the limit to the resolution of pattern formation by lithography are described above. With the conventional lithography which requires transfer of a pattern on a resist once, the pattern transfer makes an obstacle to further refinement of a pattern, and a processing method for forming a pattern smaller than 10 nm is not available.
In such a present situation of technology as described above, the inventors of the present invention have demonstrated holographic pattern formation which uses an atomic beam, for example, in J. Fujita, M. Morinaga, T. Kishimoto, M. Yasuda, Nature, Vol. 380, No. 6576, pp. 691-694 (1996), M. Morinaga, M. Yasuda, T.Kishimoto, F. Shimizu, J. Fujita and S. Matsui, Phys. Rev. Lett. , Vol. 77, No. 5, pp. 802-805 (1996), and M. Morinaga, J. Fujita, S. Matsui, F. Shimizu, Oyo Buturi (J. Appl. Phys (Tokyo)), Vol. 65, No. 9, pp. 912-918 (1996). According to the atomic beam holography disclosed in the publications mentioned, a hologram of the transmission type is formed, and the hologram thus formed is used to reproduce a hologram image on an objective substrate by directly projecting the hologram image with an atomic beam to form a pattern. According to the transmission type atomic beam holography, desired atoms,(element) can be deposited on a substrate directly and without requiring a resist process. In fine pattern formation in which a conventional pattern formation technique is used, a fine pattern is formed by a process of etching or lift-off after, as an intermediate process, a pattern is transferred once to an organic or inorganic resist. In contrast, the technique of atomic beam holography is an epoch-making lithography technique in that atoms are deposited directly at a desired place to form a pattern.
However, with the lithography by a transmission type atomic beam hologram described above, the resolution is limited by the size of holes formed in the hologram for passing atoms therethrough. According to the atomic beam holography, it is possible to utilize an atomic wave, which is an atomic beam as a material wave, of a wavelength of the angstrom order in principle, and a wavelength shorter than the distance of atomic arrangement of a substance must be obtained readily. However, formation of a transmission hologram with such an atom size as just mentioned requires a pattern formation technique which can control atoms on the order of atomic arrangement, and this is really impossible. Thus, a technique which overcomes this difficult situation is required.